Internal combustion engines such as, for example, diesel engines, gasoline engines, natural gas engines, and other engines known in the art, exhaust a complex mixture of chemical pollutants. The chemical pollutants may include solid particulate matter, including hydrocarbon, and gaseous compounds, which may include nitrogen oxides (NOx) and carbon monoxide (CO). Due to increased attention on the environment, exhaust emission standards have become more stringent, and the amount of pollutants emitted to the atmosphere from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine.
One method that has been implemented by engine manufacturers to comply with the regulation of particulate matter exhausted to the environment has been to remove the matter from the exhaust flow of an engine with particulate filters. However, over time the particulate matter builds up in the filter medium, thereby reducing functionality of the filter and subsequent engine performance. To reduce the buildup of particulate matter and return functionality to the filter and engine, the particulate trap is periodically regenerated. Regeneration involves oxidizing, or combusting, the particulate matter, and is often achieved by increasing the temperature within the particulate filter with a fuel powered burner or an electrical grid. Although this method is generally successful, the combustion of the particulate matter, particularly hydrocarbon, in oxygen requires high combustion temperatures of approximately 600-700° C. Because these temperatures typically exceed the operating temperature of a diesel engine, in order to improve regeneration, it may be desirable to reduce the temperature at which the hydrocarbon combusts.
One method of reducing the combustion temperature of particulate matter is disclosed in U.S. Pat. No. 4,902,487 (the '478 patent), issued to Cooper et al. The '478 patent discloses a method of providing nitrogen dioxide (NO2) to the particulate matter in order to reduce the combustion temperature of the particulate matter. The method includes passing exhaust gas through a catalyst coated with platinum or another platinum group metal, so that nitric oxide (NO) in the exhaust gas is catalytically converted to NO2. The NO2 is fed to the particulate filter where the particulate matter is combusted in a temperature range of 250-400° C.
Although the method of the '478 patent may reduce the combustion temperature of particulate matter, the temperature range required for regeneration may still exceed the operating temperature of the engine. In order to improve regeneration, it may be desirable to further reduce the combustion temperature.
The disclosed exhaust system is directed to overcoming one or more of the shortcomings set forth above and/or other shortcomings in the art.